In a ring laser gyro measuring rotation by means of coherent light traveling in opposite directions around a loop, a difference in light frequencies arises between the two counter-circulating beams when the loop is rotated in inertial space.
The difference in the frequency is proportional to the angular rate .omega. of the loop rotation in inertial space. By superimposing or heterodyning the two light-beams, and feeding the light output to a photodetector, a beat-frequency signal is obtained, which is indicative of the angular rate .omega..
Below a certain rate threshold, however, the separate oscillations of the opposite traveling beams are no longer sustained, because of the mode-locking phenomenon. The two modes become pulled into synchronization by cross coupling arising from the scattering of the gas molecules within the laser gain medium.
Many methods have been devised to overcome or reduce the rate threshold caused by the lock-in phenomenon. The best known is biasing, like dithering. These additional correction methods, however, do not remove the original source of the error and are limited in performance. Their limitations have to be considered as the practical rate threshold of the ring laser system. If the mode-locking phenomenon itself could be removed, a laser rate sensor far superior in accuracy to any existing gyro would be feasible. The laser rate sensor could operate at the fundamental limit set by the noise process of spontaneous emission. According to the literature, this limit is so low that its detection is virtually impossible. See IEEE Spectrum Oct. 1967 "The laser gyro" by Joseph Killpatrick, Honeywell Inc., p. 51-53.
The present invention avoids mode-locking at the outset by avoiding two different light frequencies in the amplifier medium. This is achieved by utilizing the polarization of the light. The two opposite traveling linearly polarized light beams are superimposed in the gain medium in such a way as to form always linearly polarized light oscillating with a constant frequency the same as when the angular rate .omega. is zero. This is accomplished by the proper application of a simple quarter wave plate in a specific ring interferometer arrangement proposed by U.S. Pat. No. 3,692,385.
Since the laser rate sensor of the present invention avoids mode-locking altogether and at the same time any null shift errors, which may be caused by gas flow in the discharge cavity, an optical rate sensor of an exceptional high accuracy is feasible.